This invention relates to processes for bonding nonwoven webs of organic fibers to form nonwoven fabrics. More specifically, the invention relates to such processes wherein the web is preferentially bonded in spaced, discrete areas.
Nonwoven fabrics and numerous uses thereof are well known to those skilled in the art. Such fabrics are prepared by forming a web of continuous filament and/or staple fibers and bonding the fibers at points of fiber-to-fiber contact to provide a fabric of requisite strength.
Depending on the intended use of the nonwoven web, satisfactory bonding can in some instances be accomplished mechanically, e.g., by needle punching or interlacing of the fibers or by application of adhesives to the fibrous web. However, in a number of applications nonwoven fabrics bonded by autogenous fiber-to-fiber fusion are desired. Bonding of this type is in some instances obtained by the application of heat to soften or plasticize the fibers and render them cohesive. In such thermal bonding techniques the web can be subjected to mechanical compression to increase contact of the thermally softened fibers and provide bonds of required strength. When web fibers are bonded at essentially all points of fiber-to-fiber contact, for example, by overall compression of the web in the presence of heat, the resultant nonwoven fabric tends to be stiff and boardy and characterized by low elongation and tear resistance. That is, such overall bonded fabrics are frequently more similar to paper than to conventional textile fabrics. In order to more closely simulate the properties of conventional textiles, nonwoven "point-bonded" fabrics have been prepared by processes tending to effect preferential bonding in spaced, discrete areas (primary bond sites). In order to provide point-bonded nonwoven fabrics of adequate strength, it is generally necessary that bonding of the web in the primary bond sites be accompanied by mechanical compression. This is generally accomplished by compressing the nonwoven web between mechanical compression means such as a pair of rollers or platens at least one of which carries bosses sized and spaced to provide the desired pattern of primary bond sites or both of which carry land and groove designs interacting to provide the desired pattern. The compression means are generally heated sufficiently to effect thermal bonding. By a proper selection of sizing and spacing of the bosses or lands and grooves, control of the bonding conditions (temperature and compressive force), it is possible to obtain nonwoven point-bonded fabrics having acceptable strength and improved tactile properties such as softness. However, even point-bonded fabrics are frequently less soft than conventional fabrics of comparable strength. This is probably due, at least in part, to "tack" bonding. When the bonding conditions are controlled to provide fabrics having good strength and durability during washing, bonding is not limited to the primary bond sites produced in the areas compressed. Varying degrees of secondary or "tack" bonding are generally observed between the primary bond sites. Such "tack" bonding probably results from the fact that techniques employed for preparing point-bonded nonwoven fabrics expose areas of the web between the areas being compressed to heat sufficient to effect some softening and tack bonding of fibers at points of contact. The strength and number of the tack bonds formed may vary widely with the properties of the fiber utilized in the web as well as the conditions employed for effecting bonding in the primary bond sites. Desired fabric properties such as softness are progressively impaired as the degree of tack bonding is increased. There is, therefore, a need in the art for processes capable of providing softer nonwoven fabrics.